Introduces '(org-roam-title-sources), which can be a list of lists. Each symbol corresponds to a function, that when called returns a list of titles. If the element is a list, use the first function in the list that returns a successful result.
`org-roam-doctor` provides a diagnostic tool for checking an Org-roam
file for things that are broken. Currently implemented is a check for
broken links, and methods to fix them. The checker is designed to be
extensible.
